1. Field of the Invention
The present invention relates to polycrystalline magnetic substances for magnetic refrigeration for carrying out cooling by the use of magneto-caloric effect, and a method of manufacturing the same, and more specifically to polycrystalline magnetic substances for magnetic refrigeration with an excellent heat conduction property which is capable of producing a sufficient cooling effect over a wide range of refrigeration temperature region, and a method of manufacturing the same.
2. Description of the Prior Art
Accompanying the remarkable advancement in the superconduction technology which has taken place in recent years, industrial electronics is being contemplated for its application to a wide range of fields such as information industry and medical apparatus. In order to employ superconduction technology, it is indispensable to develop a refrigeration method. However, this method has a very low efficiency, and moreover, the facility required becomes large in size so that research on the magnetic refrigeration method that makes use of the magneto-caloric effect of magnetic substances has been going on vigorously as an alternative new refrigeration method (see, for example, Proceedings of ICEC 9 (May, 1982), pp. 26-29 and Nakagome ex al, in Advances in Cryogenic Engineering, 1984, Vol. 29, pp. 581-587). Nakagome et al describes a new magnetic refrigeration process for liquefying helium from the temperature of 20K, using gadolinium-gallium-garnet (GGG) for the magnetic material. The refrigerator for such a process consists of (1) a magnetic material (GGG single crystal), (2) a heat expelling portion (20K gaseous helium flow line), (3) a low temperature portion at 4.2K (liquid helium bath), (4) a heat pipe as a thermal switch, and (5) a superconducting pulsed magnet.
Nakagome's magnetic refrigeration process operates on the following principle (a) when the magnetic material is placed in a high-intensity magnetic field generated by the pulsed magnet, the temperature of the GGG rises to over 20K, (b) the temperature of the GGG is then lowered by removing heat from the GGG by flowing 20K gaseous helium over its surface, (c) after removal of heat, the magnetic field is eliminated and the temperature of GGG goes below 4.2K, and (d) the GGG then absorbs heat from liquid helium bath through the heat pipe which functions as a thermal switch on the low temperature side.
The basic principle of the magnetic refrigeration method is to utilize the endothermic and exothermic reactions due to the change (.DELTA.S.sub.M) in entropies for the spin arrangement state which is obtained by applying a magnetic field to a magnetic substance and for the state of irregular spins that is obtained when the magnetic field is removed. Since the larger the .DELTA.S.sub.M, the larger is the cooling effect obtained, various kinds of magnetic substances are being investigated.
As may be clear from FIG. 1 which shows the relationship between the temperature and .DELTA.S.sub.M for a magnetic substance, .DELTA.S.sub.M temperature (magnetic transition point) and decreases for the temperatures above and below that point. It means then that a sufficient cooling effect can be obtained for only a delicate temperature range which is in the neighborhood of the magnetic transition point for such a magnetic substance.
In order to resolve the above problem, one only needs to adopt a magnetic substance that possesses a plurality of different magnetic transition points. As a result, it will become possible to obtain a sufficient cooling effect over a relatively wide range of temperature region.
As materials that can form magnetic substances that possess a plurality of magnetic transition points, there are known RA1.sub.2 Laves type intermetallic compounds (R signifies a rare-earth element) and others (see Proceedings of ICEC 9 (May, 1982) pp. 30-33 and others).
In other words, by mixing powders of two kinds or more of such compounds and sintering the mixture, it is considered that a magnetic substance that possesses a plurality of magnetic transition points can be obtained. However, in a magnetic substance that is obtained by above method, mutual diffusion proceeds during sintering among the powders of different kinds of compound, and as a result, .DELTA. S.sub.M will become to have just one maximum.
In addition to the RA1.sub.2 Laves type intermetallic compounds, there are known garnet-based oxide single crystals represent by Gd.sub.3 Ga.sub.5 O.sub.12 and Dy.sub.3 Al.sub.5 O.sub.12 that include rare-earth elements. However, it is known that a sufficient cooling effect can be obtained only for the temperature region below 4K in these materials. Accordingly, such substances cannot respond to the demand for polycrystalline magnetic substances which can provide a sufficient effect over a wide ranges of temperature region above 4K.
For instance, in Japanese Patent Publication No. 60-204852, there are disclosed porous magnetic substances obtained by sintering the mixture of three kinds or more of magnetic substances with different Curie temperatures.
However, the magnetic substances described in the above publication are porus sintered bodies so that their heat conductivity is poor and hence it is difficult to effectively utilize the magneto-caloric effect that has advantages as described above.
On the other hand, if a magnetic substance is sintered by compacting it under high pressure in an attempt to obtain a magnetic substance with high filling factor for the powder of the magnetic substance, there is formed a homogeneous solid solution, so that such a substance has a disadvantage in that it is not possible to obtain a large entropy change over a wide range of temperature region.